Silica particles are used as resin fillers. For example, they are used as fillers for sealants of semiconductor devices. If the silica particles are shaped with angles, the fluidity, dispersability, and fill factor in the resin will become poor and, further, the manufacturing apparatus will become increasingly worn. To deal with this, spherical silica particles are broadly used.
In general, flame spraying is used as the method for producing spherical silica. With flame spraying, the particles are passed through a flame so that the particles melt and become spherical in shape due to surface tension. The melted particles made spherical are prevented from melt fusing with each other by conveying them and recovering them by a flow of gas, but the flame sprayed particles are rapidly cooled. Since the particles are rapidly cooled from the molten state, the silica does not contain much crystals and is amorphous in structure.
Since the spherical silica is amorphous, its coefficient of thermal expansion and thermal conductivity are low. The coefficient of thermal expansion of amorphous silica is 0.5 ppm/K, while the thermal conductivity is 1.4 W/mK. These physical properties are generally equal to the coefficient of thermal expansion of quartz glass not having a crystal structure, but having an amorphous structure.
A sealant filled with a high amount of amorphous silica with a low coefficient of thermal expansion is extremely low in coefficient of thermal expansion, so warping or cracks sometimes occur due to the heating temperature at the time of reflow or the operating temperature of the semiconductor devices. Further, due to the low thermal conductivity, dissipation of the heat generated from semiconductor devices is also becoming a problem.
On the other hand, as crystalline structures of silica, there are cristobalite, quartz, tridymite, etc. Silica having these crystal structures are known to have higher coefficients of thermal expansion and thermal conductivities than amorphous silica. For these reasons, various methods have been proposed for crystallizing amorphous spherical silica to raise the coefficient of thermal expansion (PLTs 1 and 2).
One of the conventional means for crystallizing amorphous silica is heat treatment of high purity amorphous silica at a high temperature followed by gradual cooling so as to promote crystallization. PLT 3 proposes to heat spherical amorphous silica at a 1200 to 1600° C. high temperature for 5 to 24 hours to make the crystals reliably grow, then slowly cool them over 20 to 50 hours down to room temperature to cause formation of cristobalite.
Further, NPLT 1 reports on the effects on crystallization and phase transition of the addition of 0.5 to 7.0 mass % of an alkali metal oxide to amorphous spherical silica and sintering the same. In the case of no addition, no crystal phases could be observed in the sintered silica. The greater the amount of addition and the higher the sintering temperature, the more crystallization was promoted.